Document Type

Dissertation

Degree

Doctor of Philosophy

Major

Physics

Date of Defense

7-23-2021

Graduate Advisor

Philip B. Fraundorf

Co-Advisor

Julia E. Medvedeva

Committee

Eric H. Majzoub

Yew San Hor

Stephen M. Holmes

Abstract

A subset of micron-size meteoritic carbon particles formed in red giant atmospheres show a core-rim structure, likely condensed from a vapor phase into super-cooled carbon droplets that nucleated graphene sheets (~40Å) on randomly oriented 5-atom loops during solidification, followed by coating with a graphite rim. Similar particles form during slow cooling of carbon vapor in the lab.

Here we investigate the nucleation and growth of carbon rings and graphene sheets using density functional theory (DFT). Our objectives: (1). explore different computational techniques in DFT-VASP for various carbon structures and compare the results with literature, (2). investigate the nucleation and growth of carbon rings and graphene sheets at the experimental 1.8 g/cc density estimate, by supercell relaxation of randomized liquid-like carbon atom clusters, and (3). Compare carbon cluster energies for combinations of DFT-VASP and long-range carbon bond order potential (LCBOP) relaxations.

Observations show: (a) that 29 atom diamond clusters relax into the C28 fullerene with a central carbon atom, (b) new evidence for the instability of an Fm3m carbon phase with the diamond unit cell, and (c) that pent-loop formation is energetically favored over hex-loop formation in a relaxed melt. Literature work on the effectiveness of pent-loops as nucleation seed for graphene structures, plus the fact that each pent-loop can give rise to 5 differently oriented sheets, helps explain electron-microscope data on graphene-sheet number densities and provides guidance for nucleation/growth models being developed.

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